The present disclosure concerns ongoing efforts in developing lactide polymers useable in preferred manners. U.S. Pat. No. 5,142,023 issued to Gruber et al. on Aug. 25, 1992, discloses, generally, a continuous process for the manufacture of lactide polymers with controlled optical purity from lactic acid having certain desired physical properties. A related process for generating purified lactide and creating polymers therefrom is disclosed in U.S. Pat. No. 5,338,822 issued to Gruber et al. on Aug. 16, 1994.
Generally, manufacturers of polymers utilizing processes such as those disclosed by Gruber et al. in the '023 and '822 patents will convert raw material monomers into polymer beads, resins or other pelletized or powdered products. The polymer in this form is typically sold to end users who extrude, blow-mold, cast films, blow films, foam, thermoform, injection-mold or fiber-spin the polymer at elevated temperatures to form useful articles. The above processes are collectively referred to herein as melt-processing. Polymers produced by processes such as those disclosed by Gruber et al. in the '023 and '822 patents, which are to be sold commercially as beads, resins, powders or other non-finished solid forms, are herein generally referred to collectively as polymer resins.
It is generally known that lactide polymers or poly(lactide)s are unstable. The concept of instability has both negative and positive aspects. A positive aspect is the biodegradation or other forms of degradation that occur when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. A negative aspect of such instability is the degradation of lactide polymers during processing at elevated temperatures as, for example, during melt-processing by end-user purchasers of polymer resins. Thus, the same properties that make lactide polymers desirable as replacements for non-degradable petrochemical polymers also create undesirable effects during production of lactide polymer resins and processing of these resins.
There are a number of technical problems which have heretofore inhibited development of commercially viable lactide polymer-based replacement resins for existing conventional resins. Lactide polymers are subject to unwanted degradation during melt processing via a number of pathways. These pathways include hydrolysis and other side reactions, which, for example, result in molecular weight decline and/or lactide formation. Furthermore, at high processing temperatures (especially to above about 230.degree. C.), lactide polymer degradation is accelerated.
Some of polylactide's physical properties make it difficult to use for particular types of applications. In general, polylactide is a relatively brittle polymer with a low impact resistance. Because polylactide is a relatively brittle polymer, articles made of polylactide may be brittle and prone to shatter under use conditions. For example, if polylactide is used to make articles such as razor holders, shampoo bottles and plastic caps, these articles may be prone to undesirable shatter in use.
There exists a need for polylactide polymers with modified physical properties, such as impact resistance, and methods for making these types of polylactide polymers preferably without unduly compromising other physical properties, such as tensile modulus, yield strength, and blocking resistance (i.e. tendency to stick).